1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method.
2. Description of the Related Art
In recent years, in a copier, a printer and the like, for the purpose of reducing an amount of data, compression processing has been generally performed in the event of storing an image in a memory. As a compression method, a block truncation coding (BTC) mode has been widely known. However, in the case where, in one image, there are mixedly present: a region for which high resolution is required, such as text and graphics; and a region for which high gradation rather than the resolution is required, such as a photograph, then it is difficult to strike a balance between the resolution and the gradation if the compression processing is performed by one compression method. In particular, it has been known that, with regard to edge portions of the text and the graphics, such an image having original resolution cannot be maintained by being subjected to the compression processing and resolution conversion, and sharpness thereof is lost.
In this connection, a technology has been disclosed, in which the BTC compression processing is used in the case of a pixel in a halftone range, and compression processing (fallback compression processing) based on a density pattern is performed in the case of a pixel in a high gradation region, whereby the compression processing is differentiated in response to whether a pixel is the pixel in the halftone range or the pixel in the high gradation region (refer to Japanese Patent Laid-Open Publication Nos. 2009-94619 and 2010-87961).
In the fallback compression processing, it is necessary to estimate and restore the original density pattern from a quantized value at the time of extension. In this estimation of the density pattern, pattern matching is performed between a pixel of interest and pixels around the pixel of interest in compressed data, whereby the density pattern after the restoration is decided. Therefore, an image to be restored by extension processing after the fallback compression is prone to be affected by such peripheral pixels, and in particular, in the graphics and minimum text, it has been possible that an original image cannot be maintained.